Metabolic disorders and genetic diseases of the brain can potentially be cured by transplantation of neural stem cells. These cells can migrate and differentiate into neurons, astroglia or oligodendrocytes and thereby can deliver therapeutic genes or even replace defective cells in the brain. A major challenge towards development of stem cell therapy has been to non-invasively monitor the migration and gone expression of these donor cells. Green fluorescence protein and luciferase are suitable only for superficial organs while herpes-simplex virus-thymidine kinase gone requires binding to acyclovir ligands that do not cross the blood brain barrier. The sensitivity of these techniques is high but spatial resolution is very limited. Stem cells labeled with iron oxide particles can be detected in vivo using magnetic resonance imaging. These cells can also be labeled with a lentiviral dopamine type 2 receptor and can be imaged by PET to monitor their gene expression in vivo. The overall goal for this proposal is to evaluate the utility of magnetic resonance imaging (MRI) and positron emission tomography (PET) for cell tracking and imaging gene expression in the brain. This hypothesis will be tested with the following specific aims: Aim 1: To serially monitor migration and engraftment of C17.2 neural progenitor cells tagged with clinically approved super paramagnetic iron oxide (SPIO) particles using high resolution in vivo MR imaging in a mouse brain. Aim 2: To image gene expression in donor C17.2 neural progenitor cells in the mouse brain using the mutant dopamine type 2 receptor (D2R-80A) and 3-(2'-[18F]-fluoroethyl)-spiperone (FESP) ligand with micro PET imaging. Aim 3: To co-register migration of C17.2 cells with gene expression of the D2R receptor using in vivo MR and PET imaging. Neural C17.2 progenitor cells will be transduced with the lentiviral D2R-80A receptor and with clinically approved iron oxide particles. C17.2 cells express the beta-galactosidase gene from a clonally integrated site which will be used as an independent donor cell marker. The cells will be transplanted intracranially in the hippocampus of adult mice and in the ventricles of neonatal mice. Migration of these cells will be monitored serially using spin echo and gradient echo MR imaging. Gone expression of the D2R receptor will be followed by its binding with FESP ligand and will be imaged using a micro PET. Correlation of MRI and PET images will be performed in vivo and in vitro using anatomical markers and with immunohistochemical staining for the beta-galactosidase activity.